Our work is concerned with the elucidation of basic principles in the development of higher organisms. The emphasis is on understanding the processes by which cells become committed, i.e., determined, to a particular developmental pathway and the processes that order various determinations in space, i.e., pattern formation. The advantages of applying genetic methodology make Drosophila an organism of choice as a model system for investigating these fundamental processes. We are continuing to investigate the developmental and cellular effect of a temperature-sensitive mutation which causes paralysis plus a broad range of anomalies, including eye defects, abortive gastrulation and other congenital abnormalities. We are trying to identify the phase of the cell cycle which is sensitive to the restrictive temperature and to develop a bioassay for the wild-type gene product. The fact that cells can grow in the absence of the shi ion gene product but do not maintain their developmental commitments indicates that we are dealing with a level of cell regulation particular to multicellular organisms. The results promise to increase our understanding of basic processes applicable to development and neoplastic growth.